1. Field of the Invention
The invention refers to an image forming device in printers of the type which comprise an electrode unit, including a screen or grid of electrodes and a plurality of apertures or passages between the electrodes, situated between at least a particle carrier and a backing electrode, whereby the passages are partly opened and closed electrostatically for the controlled transition of the pigment particles through the electrode unit.
2. Description of the Prior Art
Swedish Patent No. 8704883-1 (PCT-SE88-00653) discloses a method for developing pictures and text with pigment particles on an information carrier, directly from computer generated electric signals, without the need for these signals to be intermediately stored for temporary conversion to light energy, which is the case in the photo-conductive printers, e.g., laser printers.
These problems have been solved by bringing the information carrier into electrical cooperation with at least a screen or a grid-shaped matrix, preferably an electrode matrix, which, through control in accordance with the configuration of the desired pattern, at least partly opens and closes passages through the matrix, which is galvanically connected to a voltage source. An electric field is thus exposed through the opened passages for the attraction of the pigment particles of the toner towards the information carrier.
In this method, herein referred to as the EMS-concept, the pigment is produced by the pigment particles passing through the openings or passages in the electrode matrix. Thus, the pigment particles can touch parts of the electrode matrix during the developing process. This physical contact between the pigment particles and the isolating surface layer of the electrode matrix or other non-conducting parts of the printer device gradually can give rise to tribo (friction) charges. These charges change the electric field so that the dots produced on the information carrier (e.g., the paper) are negatively affected in shape, size, blackness and definition. Further tribo charges may cause attraction forces between the pigment particles and other parts of the printer device, so that passages are plugged as a result of the increased quantity of attracted pigment particles.
It has also been observed that electric fields can attract pigment particles in areas adjacent the passages, so that developed dots on a paper can emit pigment particles onto non-desired surfaces and other parts of the device.
In the other methods, described, for example, in GB 2108432, the pigment particles touch nonconducting parts of the device, which control the electric field pattern between the information carrier and the pigment particles, and printing quality is affected and degraded by the above-mentioned tribo charges.